1. Field of the Invention
The present relates to multi-channel receiver/transmitter calibration and local oscillator mismatch mitigation.
2. Description of the Related Art
Many wireless systems employ some form of multiple-radio signal combining to improve range and reliability in wireless systems. For example, maximal ratio combining can be employed to improve the signal to noise ratio (SNR) of signals simultaneously received from multiple antenna elements and receivers. The multiple receivers may be instantiated on a single chip or, alternatively, may be formed by duplicating analog radios. To provide optimized gains on transmit, the phase difference of the transmit waveforms can be controlled at the antenna elements. This control applies a known beam pattern on transmit based on a receive beam pattern to achieve similar gains for the wireless link on both receive and transmit.
Transmit beamforming schemes that use transmit phase calibration of the differences between the duplicated analog radios depend on a consistent phase relationship between the radios for the duration of the calibration period. Unfortunately, radio architectures that employ duplicated local oscillator generation circuits per radio can provide inconsistent phase states on initialization even when derived from a common lower frequency reference clock. The resulting uncertainty in local oscillator initialization states either requires a new transmit phase calibration per initialization or a one-time transmit phase calibration with a method for compensating for the local oscillator phase difference component of the transmit phase calibration per initialization.
For transmit beam forming some form of phase calibration of receive and transmit analog radios can be performed. A need arises for a method of calibrating the phase differences between transmit and receive analog components to achieve a controlled array pattern on transmit based on information derived on receipt. A further need arises for a method for transmit beam forming to compensate for local oscillator phase ambiguity between radios sharing a reference clock.